To estimate your cost, review your most recent utility bill to see the kilowatt-hours "kWh" used. On the left side of the bill, see the past 13 months of history. Select the percent renewable power, and enter your kWh usage and press the "Calculate" button. You will be given the kWh you would be purchasing and the dollar amount.
You can click on the "Reset Form" button and try different percentage or you can enter a new kWh and calculate again.